There is an ongoing but urgent need for lighter weight sources of electricity for many applications, such as battlefield applications, unmanned aircraft, and unattended wireless sensors. Presently, the power source that is primarily used is the lithium battery. However, the energy capability of lithium batteries, for a given weight of the battery, is limited.
It is desirable to increase the power of electrical sources by many times, thus making it possible to materially increase the operating life of electronic equipment without adding weight.
The assignee of the present invention has previously developed a high performance micropower source 10 shown in FIG. 1. The micropower source 10 includes a water reservoir 12, a water permeable membrane 13, a solid fuel reservoir 14, and a pneumatic valve 16. The water reservoir 12 may be arranged to hold, for example, a small amount of water such as a few cc. The solid fuel reservoir 14 contains, for example, a solid fuel such as a few grams of a metal hydride such as LiAlH4.
The water reservoir 12 is connected to the solid fuel reservoir 14 by way of the pneumatic valve 16. As the water in the water reservoir 12 evaporates and permeates through the water permeable membrane 13, the resulting water vapor diffuses through the pneumatic valve 16 and reacts with the fuel in the solid fuel reservoir 14 to produce hydrogen gas. The pneumatic valve 16 senses the internal hydrogen gas pressure, and regulates the diffusion of the water vapor to maintain a fixed internal hydrogen gas pressure, usually slightly above atmospheric pressure.
One or more fuel cells 18 of the micropower source 10 convert the hydrogen gas to an output voltage. For example, the fuel cells 18 may be miniature proton exchange membrane (PEM) fuel cells each having a 1 mm diameter Nafion membrane, with one side in contact with air, that converts hydrogen to a dc output voltage.
One the advantages of using water vapor over liquid water is that water vapor produces essentially a 100% complete reaction with the solid fuel without the caking and clogging typically observed with the use of liquid water.
The micropower source 10 can generate up to 1.25 Watt-hours per gram of its weight, compared to 0.25 Watt-hours per gram from typical primary lithium batteries. The stored energy (3.1 Watt-hours) of the micropower source 10 requires only about 2.5 grams of fuel weight (e.g., metal hydride and water), and the additional non-fuel components can add as little as 0.8 grams weight to the micropower source 10, so that the overall specific energy of the micropower source 10 is 0.95 Watt-Hours per gram, about four times better than a typical primary lithium battery.
FIG. 2 shows an exemplary construction of the micropower source 10 configured as a C-cell sized power generator. The micropower source 10 is housed in a cylindrical housing 20 which houses the water reservoir 12 and the solid fuel reservoir 14 such that the solid fuel reservoir 14 forms a core that is surrounded by the water reservoir 12. The fuel in the solid fuel reservoir 14 may be in the form of solid metal hydride pellets 15. The pneumatic valve 16 includes a valve disk 22 that cooperates with a valve seat 24 to regulate the amount of water vapor diffusing from the water reservoir 12 to the solid fuel reservoir 14. The valve disk 22 is coupled to a valve diaphragm 26 by way of a valve stem 28 so that, as the internal hydrogen gas pressure changes, the diaphragm deflects to move the valve disk 22 in relation to the valve seat 24 to regulate the amount of water vapor diffusing from the water reservoir 12 to the solid fuel reservoir 14.
The fuels cells 18 are in contact with air and hydrogen. Porous barriers 21 hold the water in the water reservoir 12 and the solid metal hydride pellets 15 in their proper positions.
FIG. 3 summarizes the chemical processes in the micropower source 10.
The present invention relates to a power source which provides higher power levels, specific energies, and/or energy densities.